A blow out preventer is a safety device which is mounted to a subsea wellhead and is adapted to seal the wellhead in an emergency situation. Conventional blow out preventers (BOPs) generally have a number of wellbore control devices such as pairs of opposed rams which lie in a common plane, either side of the wellbore. In the event of an emergency, the rams are pushed together to seal the wellbore. The rams can be provided with cutting surfaces adapted to cut through wellbore equipment which may be located between the rams, such as a riser and/or a tool string. Once the rams have severed through the wellbore equipment, and are engaged with each other, the wellbore is sealed.
BOP rams can be controlled in one of two ways. The first is to close the rams using a pressure applied from surface, for example hydraulic pressure, to a pair of stems, each stem applying a push force to one of the rams. The drawback of such a system is that if the surface pressure fails, for whatever reason, the rams cannot be shut.
The second method is to use pressure applied from surface to hold the rams open and provide biasing devices such as springs or nitrogen accumulators, to shut the rams. When it is desired to shut the rams, the surface applied pressure is released and the biasing devices shut the rams. However, a failure of the hydraulic pressure system will result in closure of the rams unnecessarily. Additionally, biasing devices of sufficient strength to sever a riser and/or a tool string are extremely heavy, and should the biasing devices seize, the rams will not shut.
In both cases, in the open position, the ram stems extend out of the BOP housing into the surrounding water. If the hydrostatic or external pressure on the seabed is greater than the pressure in the BOP (wellbore pressure), the external pressure will act on the ends of the rams to close the rams. This pressure can be utilised to assist in sealing the wellbore. Once the rams are closed, the external pressure is balanced by one ram acting on the other. However, the most dangerous situation in a subsea well is when the internal pressure is higher than the external pressure. In this case, the internal pressure will hinder the closure of the rams, and indeed act to force closed rams apart.
Alternative BOP designs utilise a pressure balanced configuration. For example, one known type includes a pair of gates which, when it is desired to seal the wellbore, are pulled across the throughbore in opposite directions. In this arrangement, rather than lying in the same plane, the gates lie in parallel planes and slide across each other, shearing through wellbore equipment in the throughbore, to form a double barrier in the wellbore. There is a stem at each end of each gate, and the stems extend from the BOP housing into the surrounding water. The external pressure applied on the end of one stem is cancelled out by the equal external pressure applied to the end of the stem at the opposite end of the same gate. Therefore each gate is balanced at all times and the wellbore and environmental pressures have no effect on the movement of the ram.
However, this design of BOP still suffers from the same problem as the BOPs which incorporate opposed rams, in that closure of the throughbore is dependent on the presence of an external closure force in the form of an applied pressure from surface or an integral biasing device. Particularly, conventional BOPs do not have the ability to self close in a high internal pressure situation.